Dynamics of an unconstrained droplet in the transitional boiling regime

The impact of droplets on heated surfaces and subsequent droplet dynamics plays a crucial role in several applications, including spray cooling, solidification, and pharmaceuticals. At wall temperatures above the boiling point of the liquid, boiling initiates, and the regime is dependent on the wall superheat. The transition from vigorous boiling to conduction-limited film boiling regime is marked by intermittent solid-liquid contact. This transition boiling regime is characterized by an increase in the total evaporation time of the droplet with an increase in substrate temperature, unlike the evaporation and nucleate boiling regimes where the total evaporation time decreases with an increase in substrate temperature. We show that an unconstrained droplet in the transitional boiling regime undergoes trampolining behavior and high rebounds. The droplet dynamics result in a non-monotonic variation in the total evaporation time with temperature– an observation that is absent for a constrained droplet. The high rebound droplet height in the transition boiling regime is attributed to the nucleation and escape of the bubble at the liquid-substrate interface that results in a gain in momentum of the droplet before take-off.